The present disclosure relates to blind fasteners having a large bearing surface on the blind side of the structure after installation.
The Composi-Lok® fastener, sold by Monogram Aerospace Fasteners (Applicant), has been in use for over 30-years. The Composi-Lok® fastener is a blind fastener that provides a large blind side upset that distributes bearing loads over a larger area, which has proven useful in modern aircraft manufacture that makes extensive use of composite laminates. Composite laminates are capable of sustaining high loads but are subject to delamination when fastened with some conventional fasteners that do not adequately distribute bearing loads. By providing a large blind side upset Composi-Lok® fasteners distribute bearing loads over a large area overcoming damage to the composite. The characteristic large blind side bearing area or “footprint” allows the fastener to impart very high clamp loads to the structure without damage to the composite laminate. This also facilitates an installed fastener that retains a high preload on the fastened joint. The original Composi-Lok® fastener is described in U.S. Pat. No. 4,457,652, issued Jul. 3, 1984 to John D. Pratt
Since the introduction of the Composi-Lok® fastener, this fastener has been modified to change different performance characteristics. The original Composi-Lok® fastener did not use a drive nut and accommodated up to 0.050″ stack up range. The installed break-off tolerance was −0.000″/+0.103″. The first change was the Composi-Lok II® fastener that added a hex drive nut. Next, the Composi-Lok IIa® fastener was developed to provide an increased effective functional grip range. While the stack up range was still 0.050″, the IIa fastener provided the required mechanical properties even if the stack up was exceeded by 0.025″ over grip or 0.025″ under grip (providing an effective 0.100″ operational range). The Composi-Lok IIa® fastener included modifications to the nose angle of the nut and also modified the length of the sleeve, which both increased blind side protrusion which also increased fastener weight.
The next development was the Composi-Lok 3 ® fastener which added finer frictional control of screw rotation to achieve a narrow break-off tolerance: +0.000″/−0.030″. The Composi-Lok 3 ® fastener is also known as the Flush Break Composi-Lok fastener. The Composi-Lok 3 ® fastener was based on the lengthened Composi-Lok IIa® fastener with the same consequence of added weight and increased blind side protrusion. This was followed by the Composi-Lite® fastener that replaced the standard A286 SS screw in a Flush Break Composi-Lok fastener with a titanium screw for installed weight savings.
The object of the improved fasteners disclosed herein is to improve several characteristics of the Composi-Lok® family of fastener while maintaining the characteristic large blind side upset, installed pre-load and the resultant characteristic performance. While these improvements are directed to the Composi-Lok® family of fastener, it should be understood that these improvements could be applied to other types of fasteners and it is not intended that the claims presented below be limited to Composi-Lok® type fasteners.
One object of the present disclosure is a blind fastener that consistently provides a flush to below flush break-off after installation to eliminate secondary shaving, milling or grinding operations to meet aerodynamic flushness requirements.
Another object of the present disclosure is to provide a reduced weight blind fastener that provides the same performance as the original Flush Break Composi-Lok® fastener.
Yet another object of the present disclosure is to provide a blind fastener that protrudes less on the blind side compared to the original Flush Break Composi-Lok® fastener.
Another object of the present disclosure is to provide a blind fastener having a titanium nut and bolt that has a break-off tolerance comparable to a bolt manufacture from A286 stainless steel.
A further object of the present disclosure is to provide a blind fastener that includes a positive mechanical lock that restricts disassembly after installation.